Conventionally, solar cells have been manufactured with monocrystal silicon wafers. The monocrystal silicon wafers, however, are very expensive due to a long period of time required for formation thereof, making the solar cells employing them very expensive as well.
In recent years, silicon solar cells having polycrystalline silicon substrates have increasingly been lowered in cost, and its production amount has considerably increased. For widespread use of the solar cells, however, further reduction in cost is required.
Currently, for such rapidly prevailing solar cells, polycrystalline silicon is mainly used as the substrates. The polycrystalline silicon is often produced by casting, as disclosed, e.g., in Japanese Patent Laying-Open No. 11-21120. In the casting technique, molten silicon in a crucible is gradually cooled from the bottom of the crucible for solidification of silicon, to obtain an ingot having long grains grown from the bottom of the crucible as its main body. This ingot is sliced into thin plates to obtain wafers available for the solar cells.
As another way of forming a thin silicon plate without the necessity of such a slicing step, lateral pulling is disclosed, e.g., in Japanese Patent Laying-Open No. 2000-302431. A receiving tank holds silicon in the molten state, and a solidifying tank placed adjacent to the receiving tank holds molten metal at a temperature lower than the silicon solidifying temperature. The molten silicon is laterally pulled, while being in contact with the molten metal in the solidifying tank, so that it is cooled and solidified into a silicon plate continuously.
In the solidifying tank, tin, tin alloy or the like, having a solidifying point lower than that of silicon, is kept at a temperature higher than its solidifying point, for solidification of the molten silicon.
As yet another method, silicon ribbon growth without the necessity of slicing has been studied for about 20 years. In particular, the RGS (ribbon growth silicon) and other techniques have attracted attention for growth of silicon at higher speed. With the RGS technique, a thin plate of silicon is formed directly from a silicon melt. The idea is to realize high-speed growth of the silicon substrate by rapid heat transfer (extraction of the heat) from the surface dose to its solidification-growth front.
With the RGS method, the bottom flat-plate portion of the crucible filled with the molten silicon is moved in a lateral direction while being cooled, to achieve rapid growth of the silicon plate (“MICROSTRUCTURAL ANALYSIS OF THE CRYSTALLIZATION OF SILICON RIBBONS PRODUCED BY THE RGS PROCESS”, I. Steinbach et al., 26th PVSC, 1997, pp. 91-93). As the bottom flat-plate portion is pulled out of the melt, silicon on the flat plate immediately after the pull-out is of a liquid phase, which is cooled down simultaneously from the two surfaces, i.e., from the under surface of the pulled out flat plate and from the surface of the silicon.
Japanese Patent Laying-Open No. 61-275119 describes another method of forming a silicon ribbon, where a surface of a water-cooled metal roll (base substrate) is immersed in the silicon melt to form a solid silicon layer on the surface of the base substrate. This method has a purifying effect, with which a solidified ribbon having purity higher than that of the molten silicon is obtained.
The above-described conventional ways of producing silicon wafers from a silicon ingot and producing a silicon plate directly from silicon in the molten state have the following problems.
Firstly, with the casting method, silicon molten in a crucible is solidified. When the silicon melt is solidified, it expands and suffers stress from the wall surface of the crucible. To improve the crystal quality while relaxing such stress and suppressing generation of cracks, it takes a long time to produce an ingot. Further, after the ingot is produced, a batch-type slicing step is required for processing the ingot into wafers. The slicing step is conducted using a multi-wire saw, for example, to form a plurality of wafers at one time. However, the slicing takes another tens of hours. As such, with the casting method, it is difficult to provide a low-cost wafer, since it takes a long time to complete the wafers, and the slice losses during the slicing step would lower the utilization efficiency of the raw material of silicon. Further, the obtained wafers differ in quality of crystals near the bottom, near the wall, and at the center of the crucible.
In the lateral pulling method as disclosed in Japanese Patent Laying-Open No. 2000-302431, the silicon melt is pulled out in the lateral direction, which is passed through the solidifying tank filled with the molten metal of tin or tin alloy, to obtain a silicon plate. According to the method, the long period of time otherwise required for producing an ingot can be saved. The slicing step is also unnecessary, preventing the slice losses. Thus, the utilization efficiency of the silicon raw material can be increased. Further, since the crystals are obtained by extraction of the heat from the surface of the molten metal in the solidifying tank, they can be oriented in one direction. However, it would be difficult to perform strict heat control, since the solidifying tank filled with the molten metal is placed adjacent to the receiving tank of the silicon melt. More specifically, while the silicon melting point is about 1420° C., the tin melting point is 232° C., and thus, the interface between the receiving tank of the silicon melt and the solidifying tank containing the tin melt will be considerably affected by the heat convection. Further, if the temperature of the solidifying tank is raised to avoid such effects of the convection, considerable impurity contamination will occur due to generation of vapor from the molten metal in the solidifying tank.
In the RGS ribbon producing method, the stable growth itself is difficult. It is described that, at the growth front of the obtained crystal, silicon solidifies and grows with the solid-liquid interface being at an angle with respect to the flat-plate surface. It is also described that the grain is of a columnar crystal perpendicular to the flat-plate surface. However, the crystals are uncontrolled, and there would be considerable differences in property among the solar cells. Thus, although inexpensive silicon plates may be produced, the cell process needs to be modified in order to improve or stabilize the properties of the resultant solar cells. This requires a complicated process, hindering production of an inexpensive solar cell.
In the another silicon ribbon producing method disclosed in Japanese Patent Laying-Open No. 61-275119, it is described that a silicon ribbon having a grain size of at least 100 μm is obtained. It however does not include any detailed explanation.
Based on the foregoing, an object of the present invention is to overcome the above-described problems, so as to provide an inexpensive wafer and a producing method thereof.